Hugh F. Barron

Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland

The Silurian–Ordovician Southern Uplands terrane occupies a key position in the Caledonian orogen, yet its genesis is controversial. Marginal-basin, backarc, and forearc tectonic regimes have all been invoked as operative at the Laurentian margin of the Iapetus Ocean. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician suprasubduction volcanic arc, a central feature of most models. However, high-precision thermal-ionization mass spectrometer U-Pb and laser-ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557 ± 6 Ma (2σ) and probably also at 613 ± 12 Ma (2σ). The complex crystallization history recorded by the zircons shows assimilation of 1043 ± 7 Ma (2σ) Grenvillian basement into the andesite magma. The fact that no zircons have been found having ages that overlap the Caradocian depositional age of the host sedimentary rocks undermines all extant terrane models. The age profile of the detrital zircons is typical of Gondwana and Avalonia. This finding has important implications for the paleogeography of the Iapetus Ocean during the Ordovician, because the zircon data require the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.

The role of geodiversity in delivering ecosystem services and benefits in Scotland

Synopsis The ecosystem services approach is now a key driver for environmental policy and conservation management both in the UK and globally. Within this, geodiversity provides or underpins many essential provisioning, regulating, cultural and supporting ecosystem services and so it is not only the inherent value of geodiversity that matters, but also its role in ecosystem functioning. Protecting geodiversity contributes to maintaining the resilience and adaptive capacity of biodiversity and supports critical ecosystem services. In addition, the analysis of palaeoenvironmental archives and geomorphological records provides a key long-term perspective on trends, rates of change and future trajectories in ecosystems and service delivery, an acknowledged gap within the Millennium Ecosystem Assessment, as well as informing adaptive management of the effects of climate change, including sea-level rise. Better integration of geodiversity and biodiversity as part of Earth system science is critical for the future-proofing of ecosystems and their services and provides opportunities and challenges for applied geoscience.

Biostratigraphy of late Llandovery (Telychian) and Wenlock turbiditic sequences in the SW Southern Uplands, Scotland

In the SW part of the Southern Uplands of Scotland the relatively thin Moffat Shale Group (late Ordovician–early Silurian) is succeeded by a thick development of Silurian greywackes, of variable turbiditic facies. This includes late Llandovery (Telychian) quartzose greywackes with interbedded thin graptolitic shales of the turriculatus and crispus biozones, in the upper part of the Gala Group, a sequencewhich is laterally equivalent to the basal part of the Hawick Group. The age of the finer-grained calcareous Hawick Group, which here includes 1. the Ross Formation, ranges from late Llandovery (turriculatus Biozone) to early Wenlock (riccartonensis Biozone). The Riccarton Group, which contains thick units of thinly-bedded siltstones and mudstones, is of Wenlock age (riccartonensis to lundgreni biozones). Within this sequence, all the biozones of the standard graptolite zonal scheme have been recognised in the area, with the exception of the crenulata Biozone of the late Llandovery (Telychian Stage) and the murchisoni and ellesae biozones of the Wenlock (Sheinwoodian Stage). Details of the graptolite biostratigraphy areclosely comparable with those of the markedly thinner sequences of northern England. Acritarchs occur throughout the sequence but are most numerous and best preserved in the Gala Group. Poorly preserved chitinozoa and spores are also present, the former occurring sporadically throughout the succession but the latter become common only in the Riccarton Group.

Valuing Geodiversity and Geoconservation: Developing a More Strategic Ecosystem Approach

Abstract This paper reviews the values and benefits of geodiversity and geoconservation in the context of the Strategic Objectives and National Outcomes in the Scottish Governments National Performance Framework, particularly in relation to: economic development; climate change adaptation; biodiversity; science and education; and recreation, health and cultural inspiration. A key challenge is to improve the integration of geodiversity in environmental policy and its implementation in order to deliver more holistic and sustainable environmental management and wider public benefits. This requires greater awareness and recognition of the key benefits and ecosystem services that geodiversity provides for society. With an overall objective of promoting integration of geodiversity into relevant policies and decision frameworks, we advocate a more strategic, ecosystem-based approach to address: the future-proofing of ecosystem services, particularly in a context of climate change and rising sea-levels; conservation and sustainable management of geodiversity both in designated sites and in the wider countryside; raising awareness of the value of geodiversity and its contribution to ecosystem services; and improving understanding of geodiversity and key knowledge gaps. Such an approach is adopted in the Scotlands Geodiversity Charter.

The Northern Belt 100 years on : a revised model of the Ordovician tracts near Leadhills, Scotland

A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone dominated sequence was deposited in a sand-rich submarine fan environment, overlying hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose dominated sequence was punctuated by the input of fresh volcanic detritus shed from a oceanic/continental island-arc situated to the W/NW of the Southern Uplands basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Southern Upland basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands broadly corresponds to ophiolite obduction in both Scotland and Ireland, possibly in response to collision of a Cambrian-early Ordovician island-arc system with the Laurentian continental margin. If this interpretation is correct then the possibility arises that the Southern Uplands-Midland Valley terranes record the dismembering of this island-arc complex within an overall transpressional regime.

Upper Llandovery–Wenlock (Silurian) palynology of the Pentland Hills inliers, Midland Valley of Scotland

Synopsis The results of a palynological study on late Llandovery–Wenlock (mid-Silurian) successions in the North Esk, Bavelaw Castle and Loganlee inliers of the Pentland Hills, Midland Valley of Scotland, are documented. Palynological assemblages from the Reservoir Formation are dominated by acritarchs, but chitinozoa, cryptospores and scolecodonts are also present. Assemblages from the lower part of the Reservoir Formation are of low diversity and abundance, but marked increases in both abundance and diversity, particularly of the acritarchs, occur in the upper 200 m or so of the formation. The reasons for the marked increases are unclear, but could reflect changing environmental conditions during deposition of the upper Reservoir Formation, with the marine palynomorphs tracking changes in the location of certain physical and chemical properties of water-masses, for example nutrient availability, salinity or temperature. There is a general decline in the abundance and diversity of acritarchs and chitinozoans above the Reservoir Formation, although both groups, together with rare cryptospores, are present in samples from the Deerhope and Wether Law Linn formations. The Henshaw Formation yielded few marine microfossils, but more abundant and diverse spores and cyptospores than the underlying formations, consistent with an upward transition from marine to prograding terrestrial facies. The microfloras from the Reservoir, Deerhope and Wether Law Linn formations are consistent with the late Llandovery age indicated by graptolite evidence. A feature of assemblages from the Reservoir Formation, particularly the more productive samples from the upper part of the formation, is the common occurrence of sphaeromorph acritarchs, Moyeria cabottii and Tylotopalla species. The common occurrence of these forms gives the palynological assemblages a distinctive character. Similar sphaeromorph–Moyeria-Tylotopalla dominated acritarch microfloras occur at about the same level (spiralis Graptolite Biozone) in the Silurian succession of the Girvan Inlier, in the Drumyork Flags Formation. They might therefore be useful for correlating upper Llandovery rocks across the Midland Valley.

Composition and provenance of the Silurian to Devonian sandstone sequences of the southern Midland Valley

Synopsis A provenance study of rocks from the Silurian inliers and Siluro-Devonian Lanark Group sandstones of the southern Midland Valley has shown that they are petrographically and compositionally similar, and were derived from the same source terrane. The clast content of these rocks indicates that before widespread Lower Devonian calc-alkaline volcanism, this source terrane included volcanic and hypabyssal igneous rocks that appear to have been associated with a wacke sandstone-dominated sedimentary sequence and a granitic igneous suite. The possibility that the source rocks for these sequences are within the Midland Valley Terrane, but now not exposed, is discussed. Minor differences in sandstone composition between the Carmichael, Eastfield and North Esk inliers are interpreted as reflecting either slight differences in the source or deposition within individual sub-basins. Comparable regional variations within the Swanshaw Sandstone Formation suggest that sub-basin architecture continued to influence sediment dispersal patterns during the deposition of the basal Lanark Group. The onset of Lower Devonian calc-alkaline volcanism coincided with a major change in sandstone composition within the Lanark Group and accompanied the replacement of the relatively small Silurian sub-basins by a single larger basin.

Mid-Wenlock acritarchs from a Silurian inlier in the Cheviot Hills NE England

Synopsis Six samples from the S margin of the Greyhound Law Inlier on the southern slopes of the Cheviot Hills yielded diverse, moderately well preserved middle Silurian palynomorph assemblages. The taxa present suggest a mid- to late Sheinwoodian or earliest Homerian age, which is in agreement with new graptolite collections of Crytograptus ellesae to Cyrtograptus lundgreni Zone age. Thick-walled Leiosphaeridia, suggestive of deposition in a hemipelagic environment, are common in the assemblages and are associated with diverse non-sphaeromorph acritarchs and some miospores that were derived from shelf areas by turbidity currents. Acritarch and spore colours are consistent with low thermal maturation, with palaeotemperatures probably in the region of 60–130°C.

Conodont biostratigraphy of the Crawford Group, Southern Uplands, Scotland

Synopsis Extensive new conodont collections from the Crawford Group, the oldest succession in the Southern Uplands of Scotland, support the previously documented biostratigraphical ages for the included formations. The Raven Gill Formation is lower Whitlandian, Arenig (comparable in age to the Dounans Limestone in the Highland Border Complex) and the Kirkton Formation is latest Llandeilian-Aurelucian, Llanvirn to Caradoc in age. It is concluded that there is a significant stratigraphical gap within the Crawford Group. The restricted and probably fault-bounded nature of the Raven Gill outcrops suggests that these may represent olistoliths within a mélange of Llandeilian-Aurelucian age. The chert-bearing succession of the Northern Belt of the Southern Uplands thus represents the juxtaposed sedimentary records of two entirely separate basins – the oldest pre-dates the Grampian assembly of the Laurentian margin, and the younger, the Northern Belt Basin sensu stricto, entirely post-dates this event.

Muellerisphaerids from the Llandovery of western mid-Wales

Muellerisphaerids are described and illustrated from Britain for the first time. The mechanical processing method used in their extraction is described. They are preserved as pyrite internal moulds and thus lack the necessary morphological detail (i.e. spines) to be identified to family level. Other microfossils occurring in the samples are also recorded.